1. Field of the Invention
This invention relates generally to an arrangement for controlling the loading of a steam turbine system after synchronous speed has been reached, and more specifically, this invention relates to a loading control arrangement for a steam turbine system having steam bypass lines around each of the turbine stages, such as in a turbine system interfaced with a high temperature gas cooled reactor (HTGR), in which a drive turbine for the HTGR cooling system is included in the main turbine steam path.
2. Description of the Prior Art
In some types of steam turbine applications, it is desirable to keep the steam flow going through the system even when the turbines are not being driven to produce a power output. Thus, in this type of turbine system, it is necessary to provide bypass lines for the power turbine stages when power is not being extracted from the system. A so-called "European type" steam turbine bypass arrangement is utilized for starting some types of turbine systems.
Another, and very important, type of bypass arrangement is that utilized with a steam turbine system interfaced with a nuclear reactor, such as a high temperature gas cooled reactor (HTGR). In such a system, the steam generation, superheating and reheating of the steam is achieved in the HTGR, while a turbine for circulating coolant in the HTGR cooling system is driven by the steam flow through the turbine system. In order to provide a drive flow for the circulator turbine even when power is not being extracted from the system, a main steam bypass line is connected across the high pressure turbine stage, as well as a throttle valve and a control valve connected in the flow path of the high pressure turbine. Also, a hot reheat bypass line is connected across the intermediate pressure and low pressure turbine stages, as well as a reheat stop valve and an interceptor valve connected in the flow path of the intermediate and low pressure turbines.
When the turbine system has been brought to operating speed (in the case of a generating system this would be the speed necessary to drive an electrical generator to produce an electrical signal having a frequency in synchronism with the power line frequency, known as "synchronous speed"), the system is ready for loading. At this time, practically all of the steam flows through the bypass lines. Normally, a minimum flow of about 25% of the steam generated in the HTGR must be maintained to prevent steam generator instability and to drive the coolant circulator turbine. Therefore, prior to loading 25% of the HTGR generated steam is passing through the main steam bypass line, with the exception of approximately 3% to 4% passing through the main turbine to maintain synchronous speed. Also, this same amount of steam (approximately 25% of that generated) flows through the hot reheat bypass line, except for about 3% to 4% flowing directly to the intermediate and low pressure turbines through the hot reheat bypass line across the reheat stop valve and interceptor valve. This small flow through the intermediate pressure and low pressure turbines is for rotor cooling purposes in the low pressure turbine and turbine rotor heating in the intermediate pressure turbine.
In order to provide the desired power capabilities after synchronous speed has been achieved, it is desirable to transfer the steam flow through the bypass lines to the turbine stages as quickly as steam turbine thermal conditions and power system load demands permit. Loading is achieved by increasing the steam flow through the turbines while reducing steam flow through the bypass lines thereacross. To achieve this as quickly and efficiently as possible, it is necessary to simultaneously adjust the steam flow through the high pressure and intermediate and low pressure turbines, together with the bypass lines across these turbines.
One way of accomplishing this control is to mechanically gang the main steam control and interceptor valves so that these valves will be simultaneously opened or closed an equal or proportional amount. In addition, the valves in the main steam bypass and hot reheat bypass lines would also be ganged to close the valves in these lines to reduce the steam flow through the bypass lines by an amount equal to the increase in steam flow through the turbine stages. The problem with this approach is that if the valves utilized are not properly designed to perfectly match, the resulting non-linearity in the system may cause the power output generated by the system to vary considerably from that required. The design and manufacturing of the valves to minimize this error is extremely difficult. Further, even after the design problems have been overcome, there is always the chance that uneven wear or damage to the control system may introduce other non-linearities and corresponding undesired errors in the power generation.